The present invention relates to a filter for processing composite signals and a dropout compensator utilizing the fiter and, more particularly, to a filter for processing digital representations of composite signals and a digital dropout compensator utilizing the filter for separating components of a composite signal.
In systems for processing composite signals, it is often desirable to separate various signal components into different signal paths for individual processing and then recombine the processed components into a composite signal form for further use. Known devices for separating composite signals composed of different frequency components are comb filters. Generally, comb filters are known as multiple bandpass filters designed to pass signals of selected frequency bands and to reject signals outside the selected frequency bands. For example, in processing color television signals, comb filters are widely utilized for separating the luminance and chrominance components. One such comb filter produces the desired separation by processing discrete digitized samples representing the analog color television signal and providing a weighted average of three samples taken from three consecutive horizontal lines of the same field at vertically aligned picture elements of the lines. The averaging is repeated for all picture elements. While this method is applicable to analog signals, it is particularly suitable for digitally encoded signals, since these typically represent samples of discrete signal values occurring at a known sampling frequency. However, to obtain samples from the vertically aligned picture elements often requires selection of special sampling rates or special sampling phases and the addition of further sample processing circuits, thus complicating the apparatus.
For example, in some standard color television signal systems, the color subcarrier signal component differs in phase on adjacent lines of the television field raster. In NTSC systems, the phase difference is 180 degrees; and , in PAL and PAL-M systems, it is 90 degrees. Some digital signal processing systems for such television signals utilize a sampling signal frequency that is an odd multiple of the subcarrier signal frequency. To obtain samples of such television signals corresponding to locations of vertically aligned picture elements, the aforementioned further circuits are necessary to provide line-to-line phase adjustment of the sampling of the television signal. If samples corresponding to locations of vertically misaligned picture elements are comb filtered, picture distortion results. As an example, a circuit for generating samples corresponding to vertically aligned picture element locations within a television field raster is described in U.S. Pat. No. 4,075,656. The above patent reveals that such circuit contributes to the complexity of the overall system for processing digitized colored television signals. A prior art digital comb filter circuit suitable for NTSC systems is described in U.S. Pat. No. 4,143,396. While this type of comb filtering is suitable for NTSC systems, the specific embodiments described in the U.S. Pat. No. 4,143,396 must be provided with further processing circuitry for other color television systems, such as PAL, PAL-M, etc., where the different chrominance component line-to-line phase characteristics create additional problems in obtaining samples corresponding to locations of vertically aligned picture elements of consecutive lines.
Color television signal dropout compensators are examples of television signal processing devices in which the above problems are encountered. Dropout compensators are widely utilized in systems for processing color television signals, such as for magnetic recording and reproduction, to replace a deficient or missing portion of the color television signal information, commonly called a "dropout", due to unpredictable instantaneous malfunction of the system or to diminutive defects of the recording medium. When such dropouts occur in the television signal, they produce visible disturbances in the displayed picture. Dropout compensators reduce the disturbing effect of dropouts seen by the viewer.
Most prior art analog dropout compensators employ an R. F. envelope level detector which monitors the amplitude level of the modulated television signal carrier waveform. A switch normally applies the incoming continuous television signal to an output terminal of the compensator. A delay line is arranged in the television signal path, for example, between an input terminal of the compensator and an input terminal of the switch or between the output terminal of the compensator and the input terminal of the switch. When a drop in the R. F. envelope level is detected, the switch is controlled to apply the delayed signal to the output terminal, instead of the incoming television signal. The delayed signal thus replaces the deficient information. When the R. F. envelope level returns to normal, the switch is controlled to switch its input from the delayed signal back to the incoming television signal, which is then applied to the output teminal. As an example, a prior art analog dropout compensator of the above type is descried in the U.S. Pat. No. 2,996,576.
There are known analog dropout compensators utilized in color television systems which separate the continuous color television signal into the luminance and chrominance components, delay the components by one or two television line periods and invert the chrominance component on consecutive lines to assure its proper phase when substituted for a dropout in the color television signal. However, in color television systems in which the signal is in the form of digitized samples, utilization of digital comb filters for separating the components often requires use of the aforementioned additional complex signal processing circuits to obtain samples corresponding to vertically aligned picture element locations throughout each field of the television signal.
An example of another prior art digital dropout compensator is described in the manual "AVR-2 Video Tape Recorder, Theory of Operation", Catalog No. 18009179-01, published by Ampex Corporation, November, 1977, pages 9-10, 9-14, 9-20 and 9-77 to 9-92. This particular dropout compensator replaces individual digital samples of data or an entire line of data with the corresponding data from an earlier occurring line of the same field. The dropout signal to be substituted for the deficient information is stored alternately on a line-by-line basis in one of two 256-bit shift registers forming a two-line delay circuit. While the data for one horizontal line is being written into one of the shift registers, the data from two lines earlier is being read out from the same shift register. In this latter application, the chrominance and luminance signals are not separated for processing. However, the color television signal information replacing the dropout signal is delayed by two lines of the same field. The interlacing property of typical television signals results in the delayed signal appearing in the display of the television signal at a location four horizontal line positions away from its real time position. In some cases, the display of the dropout compensated signal is quite disturbing to the eye, especially if shapr vertically-oriented patterns are represented on the screen. Such vertically-oriented patterns will be horizontally displaced in the dropout compensation lines relative to the adjacent undelayed lines.